Academic literature on the topic 'Electron-phonon mediated many-body states'

The implementation of high-fidelity multi-ion entangled states and quantum gates are the basis for ion trap quantum computing. There are developed quantum gate experimental schemes for realizing multi-ion entanglement and quantum gate, such as Mølmer-Sørensen Gate and Cirac-Zoller Gate; In recent years, there are also ultrafast entanglement gates that operate outside the Lamb-Dicke regime by designing ultrafast pulse sequences. In this typical many-body quantum system, these entanglement gate schemes all couple the spin states between ions by driving the phonon energy level or motion state of the ion chain. To improve the fidelity of quantum gates, they all use modulated laser pulses or appropriately designed pulse sequences to decouple the multi-mode motion states. In this review, we summarize and analyze the essential aspects of the realization of these entanglement gate schemes from the theories and experiments, and we also reveal the basic physical process of realizing quantum gates through nonlinear interactions in non-equilibrium processes by driving ion chain motion states.

Recently, the search of superconducting materials with topological states has attracted extensive interest due to their exotic properties. By using first-principles calculations, we predict that RuC monolayer is a two-dimensional topological insulator (TI) and shows a TI–superconductor transition under electron doping, leading to a superconducting transition temperature Tc of 1.4 K. Further analysis reveals that the emergence of superconductivity in RuC depends critically on the existence of flatband optical phonons as well as the appearance of multiple electron-pockets and phonon mode softening induced by doping. Moreover, we find that Li-intercalated RuC (LiRuC) is a thermal dynamically stable, superconducting material with a high Tc of 9.8 K, benefitting from the strong electron–phonon coupling. Many other superconductors with flat phonon bands are also predicted via elemental substitution in LiRuC. Our results will broaden the research interest in exploring more superconductors and modulating their physical properties through flat phonon bands.

Two-dimensional semiconductors inside optical microcavities have emerged as a versatile platform to explore new hybrid light–matter quantum states. A strong light–matter coupling leads to the formation of exciton-polaritons, which in turn interact with the surrounding electron gas to form quasiparticles called polaron-polaritons. Here, we develop a general microscopic framework to calculate the properties of these quasiparticles, such as their energy and the interactions between them. From this, we give microscopic expressions for the parameters entering a Landau theory for the polaron-polaritons, which offers a simple yet powerful way to describe such interacting light–matter many-body systems. As an example of the application of our framework, we then use the ladder approximation to explore the properties of the polaron-polaritons. Furthermore, we show that they can be measured in a non-demolition way via the light transmission/reflection spectrum of the system. Finally, we demonstrate that the Landau effective interaction mediated by electron-hole excitations is attractive leading to red shifts of the polaron-polaritons. Our work provides a systematic framework to study exciton-polaritons in electronically doped two-dimensional materials such as novel van der Waals heterostructures.

Relocation of electronic charge plays a key role for functional processes in condensed-phase molecular materials. X-ray diffraction with a femtosecond time resolution allows for spatially resolving transient atomic arrangements and charge distributions [1]. In particular, time-dependent spatial maps of electron density have been derived from x-ray powder diffraction patterns measured with a 100 fs time resolution. In this talk, new results on electron dynamics in transition metal complexes and on field-driven charge relocations in elementary ionic materials will be presented. Crystals containing a dense array of Fe(II)-tris(bipyridine) complexes and their PF6 counterions display pronounced changes of electron density that occur within the first 100 fs after two photon excitation of a small fraction of the complexes [2]. Electron density maps reveal a transfer of electronic charge from the Fe atoms and - so far unknown - from the PF6 counterions to the bipyridine units. The charge transfer displays pronounced Coulomb-mediated many-body features, affecting approximately 30 complexes around the directly excited one. As a second topic, electron relocations induced by strong external optical fields will be discussed [1,3]. This interaction mechanism allows for generating coherent superpositions of valence and conduction band quantum states and inducing fully reversible charge dynamics. While the materials LiBH4 and NaBH4 display electron relocations from the (BH4)- ions to the neighboring Li+ and Na+ ions, LiH exhibits an electron transfer from Li to H. The latter is a manifestation of electron correlations and in agreement with theoretical calculations.

Novel many-body and topological electronic phases can be created in assemblies of interacting spins coupled to a superconductor, such as one-dimensional topological superconductors with Majorana zero modes (MZMs) at their ends. Understanding and controlling interactions between spins and the emergent band structure of the in-gap Yu–Shiba–Rusinov (YSR) states they induce in a superconductor are fundamental for engineering such phases. Here, by precisely positioning magnetic adatoms with a scanning tunneling microscope (STM), we demonstrate both the tunability of exchange interaction between spins and precise control of the hybridization of YSR states they induce on the surface of a bismuth (Bi) thin film that is made superconducting with the proximity effect. In this platform, depending on the separation of spins, the interplay among Ruderman–Kittel–Kasuya–Yosida (RKKY) interaction, spin–orbit coupling, and surface magnetic anisotropy stabilizes different types of spin alignments. Using high-resolution STM spectroscopy at millikelvin temperatures, we probe these spin alignments through monitoring the spin-induced YSR states and their energy splitting. Such measurements also reveal a quantum phase transition between the ground states with different electron number parity for a pair of spins in a superconductor tuned by their separation. Experiments on larger assemblies show that spin–spin interactions can be mediated in a superconductor over long distances. Our results show that controlling hybridization of the YSR states in this platform provides the possibility of engineering the band structure of such states for creating topological phases.

Many body quantum dynamics of phonons is steadily developed by considering the various effects of anharmonicities, defects (consider as doping or impurity concentration) and electron–phonon interactions in model Hamiltonian (instead of BCS Hamiltonian) for a high-temperature superconductor (HTS). This enables to obtain the expressions for the renormalized phonon spectrum, the renormalized phonon density of states (RPDOS). The RPDOS can be resolved into diagonal and nondiagonal parts where the nondiagonal component is found highly impurity-dependent. Considering the suitable Born–Mayer–Huggins (BMH) interaction potential, the renormalized phonon spectrum, RPDOS and generalized phonon density of states (GPDOS) of the La[Formula: see text]Sr[Formula: see text]CuO4 layered superconductor have been numerically analyzed and it was found that these quantities depend on doping concentration, anharmonicities, and temperature. The results are compared with the inelastic neutron scattering experimental data of GPDOS for La[Formula: see text]Sr[Formula: see text]CuO4 and are found in good agreement. The ratio of deviation in GPDOS to GPDOS at critical temperature ([Formula: see text] K) shows the implicit difference at [Formula: see text]. The impact of defects, anharmonicities, and electron–phonon interactions in the cuprate superconductors virtually modify the scenario of GPDOS and affirm a large number of exotic peaks in the spectrum.

Independently contacted coupled quantum wells separated by barriers which allow significant interlayer interactions but no tunneling have been fabricated. When current is passed through one layer, the interlayer interactions drag carriers in the second layer, resulting in a voltage response (for open circuits). The magnitude of the response gives a quantitative measure of the effective interlayer interactions and response functions of the system, and hence this is an excellent laboratory for the study of many-body phenomena in two-dimensional electron gases. We review the Boltzmann and Kubo formalisms for the theory of drag effects in coupled quantum wells and discuss three specific cases where many-body effects significantly affect the drag: (1) acoustic phonon-mediated drag, (2) large enhancements due to coupled plasmon modes, and (3) interplay of screening and Landau levels in large magnetic fields.

The single-particle spectral function measures the density of electronic states in a material as a function of both momentum and energy, providing central insights into strongly correlated electron phenomena. Here we demonstrate a high-resolution method for measuring the full momentum- and energy-resolved electronic spectral function of a two-dimensional (2D) electronic system embedded in a semiconductor. The technique remains operational in the presence of large externally applied magnetic fields and functions even for electronic systems with zero electrical conductivity or with zero electron density. Using the technique on a prototypical 2D system, a GaAs quantum well, we uncover signatures of many-body effects involving electron-phonon interactions, plasmons, polarons, and a phonon analog of the vacuum Rabi splitting in atomic systems.

The potential problem of anharmonic lattice vibrations in high-temperature superconductors (HTS) using the most suitable Born–Mayer–Huggins (BMH) potential has been taken up to investigate the renormalized phonon density of states (RPDOS). In order to develop the suitable results, the many body theory using quantum dynamical approach of Green’s function (GF) via an almost complete Hamiltonian (without using BCS type Hamiltonian) has been incorporated which includes harmonic electron and phonon Hamiltonian, electron–phonon interaction Hamiltonian, anharmonic and defect Hamiltonian. The derivation of anharmonic phonon GF for modified form of BMH potential enables to evaluate the renormalized and perturbed mode phonon frequencies of solids. The expressions obtained for the RPDOS can be resolved into diagonal and nondiagonal parts of which the nondiagonal part chiefly depends on impurities and disappears in pure crystals. A large number of new features are investigated in the light of this formulation followed by the numerical analysis for the energy spectrum of representative cuprate HTS YBa2Cu3O[Formula: see text]. The inclusion of defects, anharmonicities and electron–phonon interactions in the cuprate superconductors substantially modifies the scenario of RPDOS and reveals a large number of unexplained peaks in the spectrum.

Hydrogen-rich superhydrides are promising high-Tc superconductors, with superconductivity experimentally observed near room temperature, as shown in recently discovered lanthanide superhydrides at very high pressures, e.g., LaH10 at 170 GPa and CeH9 at 150 GPa. Superconductivity is believed to be closely related to the high vibrational modes of the bound hydrogen ions. Here, we studied the limit of extreme pressures (above 200 GPa) where lanthanide hydrides with large hydrogen content have been reported. We focused on LaH16 and CeH16, two prototype candidates for achieving a large electronic contribution from hydrogen in the electron–phonon coupling. In this work, we propose a first-principles calculation platform with the inclusion of many-body corrections to evaluate the detailed physical properties of the Ce–H and La–H systems and to understand the structure, stability, and superconductivity of these systems at ultra-high pressure. We provide a practical approach to further investigate conventional superconductivity in hydrogen-rich superhydrides. We report that density functional theory provides accurate structure and phonon frequencies, but many-body corrections lead to an increase of the critical temperature, which is associated with the spectral weight transfer of the f-states.

O objetivo deste trabalho é examinar os efeitos causados pela interação elétron-fônon em pontos quânticos semicondutores polares. Primeiramente, nós apresentamos cálculos detalhados da taxa de espalhamento e do tempo de relaxação eletrônico em pontos quânticos simples (Single Quantum Dot - SQD) e em dois pontos quânticos acoplados (Coupled Quantum Dots - CQDs) devido à interação entre o elétron e os fônons longitudinais acústicos (LA) na presença e na ausência de campos externos, magnético ou elétrico. O regime de energia usado no cálculo do espalhamento eletrônico foi escolhido de forma que os fônons LA dominam o processo de espalhamento. Nós verificamos que na ausência de campo externo, a taxa de espalhamento do elétron por fônons LA entre dois níveis específicos é essencialmente determinada pela diferença de energia entre estes dois níveis. Observamos que um campo magnético modula fortemente a taxa de espalhamento. Verificamos que o processo de relaxação via multicanais desempenha um papel essencial no mecanismo de relaxação do elétron de estados excitados para o estado fundamental. Um campo magnético externo aumenta ainda mais a relaxação através de transições indiretas. Também fizemos um estudo teórico dos efeitos da interação elétron-fônons longitudinais ópticos (LO) em dois pontos quânticos acoplados compostos de InAs/AlInAs. Fizemos cálculos para o polaron ressonante num regime onde a energia de confinamento do elétron é comparável a energia do fônon L0 utilizando o formalismo da função de Green e teoria de perturbação considerando temperatura zero e finita. Observamos uma renormalização do estado fundamental obtida devido a absorção de fônons virtuais para uma temperatura T > O. Discutimos os efeitos do tunelamento entre os pontos quânticos e a sua influência nas propriedades eletrônicas e analisamos o espectro de absorção óptica neste sistema. Verificamos modificações nos orbitais eletrônicos como resultado direto do tunelamento assistido por fônons. Finalmente, avaliamos os efeitos da interação elétron-fônons L0 na densidade de estados do elétron confinado em pontos quânticos utilizando dois modelos distintos: Um modelo não-perturbativo e o formalismo da função de Green. Estudamos cada método separadamente e avaliamos a densidade de estados como função da temperatura e do confinamento lateral. Consideramos um sistema com apenas dois níveis eletrônicos de energia e comparamos os dois métodos avaliando as suas diferenças básicas. Utilizando o método não-perturbativo fizemos cálculos da densidade de estados para um regime de acoplamento forte entre o elétron e os fônons LO
The purpose of this work is to study effects of electron-phonons interactions in polar semiconductor quantum dots. Firstly, we present a detailed calculation on the electron-LA-phonon scattering rates and electron relaxation processes in single and coupled quantum dots in the absence and in the presence of external magnetic or electric fields. In the absence of external field, interplay among the effective confinement lengths in different directions as well as the phonon wavelength leads to a strong oscillation of the LA-phonon scattering rate between two levels. In other words, the scattering depends strongly on the geometry and confinement potential of the quantum dot. An external magnetic field also strongly modulates the scattering rate in severa1 orders of magnitude. The magnetic field induced effects are very similar in single quantum dot (SQD) and coupled quantum dots (CQDs) where the effective confinement strength in the x-direction affects strongly the scattering rate. However, we find that the multiple relaxation process plays an essential role for electron relaxing from the excited states to ground state both in single and coupled quantum dots. Including all possible relaxation channels, an external magnetic field enhances the relaxation through indirect transitions. Secondly, we present a theoretical study on the effects of electron-LO-phonon interaction in two coupled stacked InAs/InAIAs quantum dots. The contribution of resonant and nonresonant electron-LO-phonon coupling to the polaron states are obtained in the framework of t he Green function formalism and the perturbation approach at zero and finite temperatures. Ground state renormalization is found due to virtual phonon absorption at T > O. Tunneling effects between the dots have been addressed and their influente on the electronic properties and optical absorption are analyzed. Topological modifications of electronic orbitals are found as a result of phonon-assisted tunneling. Finally, we investigate the effects of electron-LO-phonon interaction on the electron density of states in quantum dots using two distinct models. A non-perturbative model and the Green function formalism. Within the non-perturbative model, we consider only two electronic levels in a quantum dot interacting to LO-phonons. An exact solution is obtained for the polaron states and spectral function. We evaluate the density of states in the regime at zero and finite temperature for severa1 values of the lateral confinement. We compare the density of states obtained within the two models. Furthermore, we study the polaron effects in strong electron-LO-phonon coupling regime based on the non-perturbative model

The recent advent of two-dimensional materials has led to the discovery of materials with fascinating phase diagrams. These systems have exciting electronic phases which coexist or compete with each other. Here in this thesis, we have looked into two such electron-phonon mediated many-body states { charge density wave and superconductivity. We start with a novel, dynamically modulated quantum phase transition between two distinct charge density wave (CDW) phases in 2-dimensional 2H-NbSe2. There is recent spectroscopic evidence for the presence of these two quantum phases, but its evidence in bulk measurements remained elusive [1]. We studied suspended, ultra-thin NbSe2 devices fabricated on piezoelectric substrates - with tunable akes thickness, disorder level, and strain. We find that over a certain range of temperature, the conductance fluctuates between two precise values separated by the quantum of conductance, e2=h. These observations could be explained as arising from strain-induced dynamical phase transition between the two CDW states [2]. To affirm this, we vary the lateral strain across the device and map out the phase diagram near the critical point

A superconducting quantum interference device (SQUID) is the most sensitive magnetic flux sensor currently known. The SQUID can be seen as a flux to voltage converter, and it can generally be used to sense any quantity that can be transduced into a magnetic flux, such as electrical current, voltage, position, etc. The extreme sensitivity of the SQUID is utilized in many different fields of applications, including biomagnetism, materials science, metrology, astronomy and geophysics. The heart of a squid magnetometer is a tunnel junction between two superconductors called a Josephson junction. Understanding the work of these devices rests fundamentally on the BCS theory of superconductivity. In this chapter, we introduce the notion of local potential and confinement in superconductivity. We show how BCS ground state is formed from interaction of wave packets confined to these local potential wells. The starting point of the BCS theory of superconductivity is a phonon-mediated second-order term that describes scattering of electron pair at Fermi surface with momentum k i , − k i and energy 2 ℏ ω i to k j , − k j with energy 2 ℏ ω j . The transition amplitude is M = − d 2 ω d ω i − ω j 2 − ω d 2 , where d is the phonon scattering rate and ω d is the Debye frequency. However, in the presence of offset ω i − ω j , there is also a present transition between states k i , − k i and k j , − k i of sizable amplitude much larger than M . How are we justified in neglecting this term and only retaining M ? In this chapter, we show all this is justified if we consider phonon-mediated transition between wave packets of finite width instead of electron waves. These wave packets are in their local potentials and interact with other wave packets in the same well to form a local BCS state we also call BCS molecule. Finally, we apply the formalism of superconductivity in finite size wave packets to high Tc in cuprates. The copper electrons in narrow d-band live as packets to minimize the repulsion energy. The phonon-mediated coupling between wave packets (of width Debye energy) is proportional to the number of k-states in a packet, which becomes large in narrow d-band (10 times s-band); hence, d-wave Tc is larger (10 times s-wave). At increased doping, packet size increases beyond the Debye energy, and phonon-mediated coupling develops a repulsive part, destroying superconductivity at large doping levels.

In the last few years, the photo-redox process via single-electron transfer (SET) has received substantial attention for the synthesis of targeted organic compounds due to its environmental friendliness and sustainability. Of late visible-light-mediated difunctionalization of alkenes has gained much attention because of its step economy, which allows the consecutive installation of two functional groups across the C=C bond in a single operation. The construction of N-containing compounds has always been important in organic synthesis. Molecules containing C-N bonds are found in many building blocks and are important precursors to other functional groups. Meanwhile, C-N bond formation via the addition of the C=C double bond is gaining prominence. Therefore, considering the influence and synthetic potential of the C-N bond, here we provide a summary of the state of the art on visible-light-driven difunctionalizations of alkene. We hope that the construction of the C-N bond via visible-light-mediated difunctionalization of alkenes will be useful for medicinal and synthetic organic chemists and will inspire further reaction development in this interesting area.

This largest class of natural products, with >75 000 known structures, arises from a pair of five-carbon isopentenyl diphosphate isomers, one acting as a π-electron double bond carbon nucleophile, the other as an allylic cation electrophile in C–C bond alkylations. Isoprene/terpene chain growth thus occurs five carbons at a time in head-to-tail couplings by prenyl transferase enzymes. At both the C15 or C20 chain length stages, enzymes can carry out related head-to-head chain couplings to generate the C30 hexaene squalene or the C40 nonaene phytoene. Squalene is the precursor to cyclase-mediated conversion to tetracyclic sterol frameworks and pentacyclic plant systems, such as amyrin and cycloartenol. The C10 (geranyl-PP = monoterpene), C15 (farnesyl-PP = sesquiterpene), and C20 (geranylgeranyl = diterpene) head-to-tail coupled metabolites can undergo many variations of internal carbocation-mediated cyclizations to generate a large array of mono- to tetracyclic olefins and alcohols. The predominant animal sterol is the C27 membrane lipid cholesterol, available from the initial C30 biosynthetic tetracyclic lanosterol by oxygenative removal of three C–CH3 groups. This phase of sterol metabolism marks a shift from carbocation-based reactions, to radical chemistry by oxygenases, as nine O2 molecules are consumed. In further conversion of cholesterol to the female sex hormone estradiol, another eight O2 molecules are consumed, for a total of 17 O2 being reductively split in the metabolic traverse from lanosterol to cholesterol. Meroterpenoid assembly involves the intersection of isoprene biosynthetic machinery with polyketide- or indole-processing enzymes.

Polydiacetylenes have large optical nonlinearities due to the extension of their π-electron wave-functions which are delocalized over many polymer repeat-units. They also exhibit sub-picosecond excited-state relaxation-times due to the efficient non-radiative decay processes which are mediated by the large electron-phonon coupling in those one-dimensional conjugated systems [1]. Some vibration modes such as the single, double and triple carbon-carbon bond-stretching ones, which are strongly coupled to the electronic excited-states, have been characterized using various resonant Raman effects [1,2]. in order to gain insight into those relaxation processes, we have performed sub-picosecond time-resolved absorption spectroscopy of the exciton bleaching of a polydiacetylene thin film.

Organic molecules and polymers have long been known to exhibit large values of the third-order susceptibility, χ(3). However, in spite of many investigations, the mechanisms governing the nonlinear optical response of these materials are not well understood. Recently, tremendous progress in two related areas has made this problem ripe for further examination. Intense study of polyacetylene and related materials has led to a more complete understanding of the ground state and elementary excitations of conjugated polymer chains,1 while studies of quasi-2D inorganic semiconductors, notably GaAs-AlGaAs multiple quantum well structures (MQWS), have highlighted the importance of reduced dimensionality for optical nonlinearity.2 In this work we have extended the experimental and theoretical approach recently applied in MQWS to quasi-1D polydiacetylene, with unusual and novel results.

Surface mediated electron transfer is the most ubiquitous of all surface reaction types and forms the basis of electrochemistry and many imaging technologies (photography, xerography). This process also holds great promise as a simple system for efficient solar energy conversion. Providing interfacial charge transfer processes can be made to occur competitively with thermalization dynamics, it should be possible to store energy as chemical potential at hybrid semiconductor/molecular junctions and avoid heat losses in conventional solid state solar cells (and thereby double theoretical efficiency limits). This specific mechanism is referred to as the hot electron model for semiconductor photochemistry [1] (Fig.1) and requires that the electron transfer occur in the strong coupling or adiabatic regime. The degree of electronic coupling between a discrete molecular state adsorbed to the surface and the highly delocalized band states of the single crystal is the key fundamental issue. In addition, the dynamics of interfacial charge transfer have to be quantified relative to the electron thermalization dynamics of field accelerated electrons (≤ 1 eV above the CBM) which are the dominant source of photoinduced hot electrons at semiconductor liquid junctions.